Impedance analysis and protonic conduction mechanism in RbH2PO4/SiO2 composite systems

The protonic conductivity of (1 − x)RbH2PO4–xSiO2 composites prepared by mechanical milling has been investigated in the humid atmosphere. In comparison with pure RbH2PO4, the conductivity enhancement was observed and it was strongly dependent on the amount of SiO2 additives. At low-temperature (LT) phase, the conductivity of composites increased with the addition of SiO2 till x = 0.6 at which the maximum conductivity reached. At high-temperature (HT) phase, it was approximately equal to that of pure RbH2PO4 and almost kept unchanged till x = 0.5. Therefore, the jump in conductivity at ∼280 °C (superprotonic phase transition temperature of RbH2PO4) tended to decrease as x increased and finally disappeared. Furthermore, the conductivity of both LT and HT phases went down at large x values. The conductivity enhancement has been confirmed not to be influenced by absorbed water on the surface of SiO2 particles. In fact, it is due to the formation of structurally disordered or amorphous interface phase. Also, we summarized the different theoretical models of the conductivity enhancement in composites.